The present invention is related to the manufacture of fine papers, including printing and writing papers, paperboard, and most particularly, to fine papers that are coated. For such products, printed smoothness is a significant characteristic which can yield a competitive advantage.
In the past, papermakers have used a number of different techniques to improve smoothness, including fiber selection and treatment, new forming methods, pressing, calendering, size press treatments, coating and finishing. It has also been found that a substantial improvement in smoothness can be achieved by pressing the paper web in a moist state, as for example, by press drying. An example of such a process is disclosed in the article, "The Hydra-Nip Dryer: The Easy Way to Surface Smoothness", by Marc Foulger, September 1988 Tappi Journal, pages 73-76. The same process is also disclosed in U.S. Pat. No. 4,596,633. The process is designed to achieve surface smoothness on paper comparable to the smoothness which can be obtained by calendering, without the degree of compaction of the web which occurs during calendering. Another method for achieving a smooth paper surface involves the densification of the paper rawstock. Wet densification adds strength and smoothness to the web, and increases the density of the web, while dry densification reduces the strength of the web while adding smoothness. However, the smoothness achieved by dry densification is partially lost due to rewetting of the web as, for example, in the size press or coaters. Examples of wet densification include U.S. Pat. No. 3,647,618, which discloses a method for densifying and smoothening a web while improving its formation by advancing the web at 18-40% moisture through a pressure nip formed between two covered rolls (which are slightly compressible and resilient); U.S. Pat. No. 3,124,504, which discloses a method for smoothening and glossing a paper web by passing the web at a moisture content of about 50% or less through a nip formed between a resilient roll and a heated finishing roll (soft calendering nip); U.S. Pat. No. 5,316,624, which discloses the soft calendering of a web using one hard roll and one soft roll at a moisture content of from about 15-55% to increase the density of the web by at least about 10% (soft calendering nip); and, British patent No. 522,196, which discloses a method for manufacturing coated paper from a consolidated and smoothened web wherein the web is passed through a nip formed between two chilled iron rolls (hard calendering nip) at a nominal pressure of about 700 psi at a moisture content of about 35-38%. However, none of the prior art is related to the specific results of wet densification achieved by the process of the present invention specifically, the use of a hard calendering nip at high pressure. In this context, U.S. Pat. No. 5,316,624 defines a hard calendering step as comprising passing a paper web between nipped rolls, the surface of each being formed of a hard, non-resilient material, and a soft calendering step as taking place between a pair of rolls wherein the surface of one roll is hard and non-resilient, while the surface of the opposed roll is made of a firm but resilient material, which is also referred to as supercalendering.
In accordance with the present invention, a process has been developed involving the smoothening, densification and consolidation of a paper web for achieving an improvement in smoothness that has heretofore not been recognized by the prior art. For the purpose of the present invention, smoothness refers to features on the surface of the paper ranging from obvious defects to those of the smallest size that can be discerned at reading distance by the unaided eye. The Bekk smoothness test has been used to assess fine scale smoothness, and the Sheffield smoothness test to assess coarse scale smoothness.
With respect to the permanence of a smoothening treatment applied to a paper rawstock, at least two phenomena tend to undo smoothening treatments, namely springback or fiber puffing when a treated and dried web is subsequently rewetted during surface sizing or coating, and heat roughening which can occur when an overly moist web is rapidly dried, as for example, in a drying oven following heat set printing. In this context, a dried web is defined as a web which has been dried to a moisture content well known to those skilled in the art for the intended purpose. A method for overcoming heat roughening on the press involving the importance of controlled drying of the web after finishing to achieve a low moisture content entering the press is disclosed in U.S. patent application Ser. No. 07/953,254, filed Sep. 30, 1992 now U.S. Pat. No. 5,425,851, granted Jun. 20, 1995, entitled "Method for Improving the Printability of Web Offset Paper", owned by the present assignee herein. Another method for producing a smooth surface on a dried web by re-wet densification, which smoothness is retained upon rewetting, is disclosed in U.S. Pat. No. 5,378,497, granted Jan. 3, 1995, entitled "Method for Providing Irreversible Smoothness in a Paper Rawstock", also owned by the present assignee herein. This patent discloses a method for applying liquid to a paper web to achieve a non-uniform moisture content or moisture gradient in the Z-direction thickness of the web to a depth of less than about one-half the thickness of the web. However, the problem of achieving smoothness and reduced springback upon rewetting, during the paper manufacturing process, where the moisture in the web is substantially uniformly distributed, has not been previously addressed. Accordingly, it is an object of the present invention to provide a method and apparatus for smoothening, strengthening and densifying a paper web on a papermachine where the web has a substantially uniform moisture content, for the purpose of achieving superior smoothness, and for reducing fiber springback upon subsequent rewetting of the web.